As the structure of a power grid is increasingly compact, stable running of the power grid appears particularly important. For example, a 3/2 connection mode is adopted for a bus of 1000 kV, 750 kV or 500 kV. The reliability of the connection mode in supplying power is higher. Even if a certain circuit breaker trips or a certain protection mal-operates, the reliability in supplying power will not be influenced. Accordingly, composite voltage locking is typically not configured for differential protection of 3/2 connection mode based bus protection. Moreover, the composite voltage locking serving as a maloperation measure of the differential protection cannot play a role in protection of the 3/2 connection mode based bus.
In a newly-built substation, the number of strings of a 3/2 connection mode bus is 2 to 4, and under most circumstances, the number is 2. If there are only 2 strings, in the case where composite voltage locking is not configured, after a branch CT is disconnected, a differential current caused by CT disconnection is equal to a load current of the other string. In such case, owing to the brake characteristic of differential protection, it is difficult to brake the differential protection, thereby causing maloperation of the differential protection due to the CT disconnection. Therefore, since composite voltage locking is not adopted for the differential protection of existing domestic 3/2 connection mode based bus protection and there exists an upper limit of the number of times for reliable tripping of a high voltage circuit breaker, how to prevent maloperation of 3/2 connection mode based bus protection during CT disconnection becomes a problem to be urgently solved.